1. Field of the Invention
This invention relates to a packet transmission system to be used in a packet communication system wherein both bursty traffic such as data and periodical traffic such as a voice signal, a video signal or a pictorial image (collectively referred to as voice hereinafter) coexist in a multiaccess channel.
2. Description of the Prior Art
In recent years much efforts have been paid to the development of a local area network system where plural computers are connected to a newtork so as to share a file and a printer or to share traffic loads in the network. Since information to be transmitted is bursty in such a system, there is often used a random-access control technique which allows nodes having data to gain access at random to a common bus for transmission of packets without establishing dedicated channels respectively among individual nodes. Various random access techniques are introduced in "Multiaccess Protocols in Packet Communication System" by Fouad A. Tobagi, IEEE Transactions on Communications, Vol. COM-28, No. 4, April 1980. The ALOHA protocol, Carrier Sense Multiple Access (CSMA) protocol, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol which are mentioned in the above article achieve high reliability and less propagation delay since communication control functions are not centralized but distributed among nodes. These random access techniques are therefore extremely effective when data is generated at burst and the amount of traffic is small.
When those conventional techniques are applied to a network system of a so-called ISDN type (integrated service digital network) or a multiaccess system where the bursty traffic and periodical traffic coexist, however, delay amounts tend to fluctuate. As a result, voice quality in the communication in which realtime processing is highly required, is deteriorated. Further, as the traffic increases, collision among packets increases to make reliable data transmission impossible.
Referring to FIGS. 1 and 2, the above-mentioned disadvantages will be described by focusing on a CSMA/CD technique-employing system disclosed in U.S. Pat. No. 4,063,220.
Referring to FIG. 1, nodes X, Y and Z connected by a bus 1 are equipped with packet transmitter/receivers 2a, 2b and 2c and terminals 3a, 3b and 3c, respectively. The terminals 3a, 3b, and 3c access the bus 1 via the transmitter/receivers 2a, 2b and 2c which function to transmit packets from the terminals 3a, 3b and 3c to the bus 1 at an appropriate timing or selectively take out of the received packets the packets addressed to these particular terminals 3a, 3b and 3c and transmit them thereto, respectively.
Referring now to FIG. 2, a format of a packet transmitted from the nodes X, Y and Z comprises 64 bits in preamble, 48 bits in destination address, 48 bits in source address, 16 bits in type field which expresses the type of data, 8n bits (n=46-1500) in the data field, and 32 bits in circular redundancy check (CRC).
The CSMA/CD protocol will be briefly explained. Out of the nodes X, Y and Z, the node Y which intends to transmit data senses whether the bus 1 is idle. If the bus 1 is idle, the node Y adds a destination address to the data signal and transmits the packet shown in FIG. 2 onto the bus 1. This packet is transmitted in both directions of the bus 1. Each of the nodes X and Z continuously monitors the destination address of the packet. If said address is destinated to its own node, the node takes the packet.
Assuming now that the nodes Y and Z have information to transmit to the node X, both nodes Y and Z sense whether the bus 1 is idle. If the node Y senses that the bus 1 is idle, a packet addressed to the node X, is transmitted from the node Y. At almost the same time, if the node Z senses that the bus 1 is idle, a packet addressed to the node X is transmitted from the node Z. Both nodes Y and Z then detect data collision, and, at the same time, abort their transmission of packets respectively, wait for an interval of a random time, and then re-transmit packets in accordance with the above-mentioned protocol.
The above-mentioned CSMA/CD protocol is quite successful to achieve a high throughput at a small probability of collision when the length of the bus 1 is short and the propagation delay can be negligibly small in comparison with the packet length. However, since the length of the bus 1 reaches a certain level, the propagation delay becomes significant, and the number of collisions suddenly increases with a concurrent high fluctuation in the waiting-time interval between the transmissions of packets.
If a certain duration of time elapses in waiting, that is handled as a packet loss in the voice communication where the requirement for the real-time processing is quite strict. Therefore, that much fluctuation will consequently degrade the voice quality.
The protocol of a so-called priority control is often utilized to avoid such inconvenience. The protocol gives a higher priority to voice packets to facilitate their transmission. However, the above-described collision unavoidably occurs among voice packets. Consequently, the waiting-time interval fluctuats to make the throughput as a whole unsatisfactory.